What unit would you use to measure the water in an aquarium?

Answer 1

That depends on which property of the water you need to measure.

-- Its temperature . . . degrees Celsius or Fahrenheit.

-- Its volume . . . liter or fluid ounce.

-- Its weight . . . Newton or pound.

-- Its mass . . . kilogram or pound.

-- Its conductivity . . . Siemens.

-- Its depth . . . inch or centimeter.

-- Its clarity or purity . . . I don't know units for either of those, but my only point

is that you don't "measure water" . . . you measure its properties.